Zero-mode microwave applicator

ABSTRACT

A fast-mode particularly a zero-mode microwave application for the heating, drying, curing or other treatment by microwave energy of workpieces containing lossy dielectric material having a fast wave microwave structure and microwave energization means connected to the structure to energize it such that a region adjacent said structure will contain a predominance of zero-mode or near zero-mode electric field energy.

United States Patent 1 1 [111 3,73Lfl38 Bosisio 1 May 1, 1973 [54] ZERO-MODE MICROWAVE 3,469,054 9/l969 Serota ..2l9/l0.6l

APPLICATOR 3,553,413 1/1971 Soulier ..219/10.55

3,590,202 6/l97l Day 6! al. ..2l9/l0.55 [75] Inventor: Renato G. Bosisio, Longueuil,

, Quebec Canada Primary Examiner-J. V. Truhe [73] Assignee: Canadian Patents and Development Assistant Exami"eHgh Jaeger Limited, Ottawa, Ontario, Canada Hughes 22 Filed: Feb. 22, 1971' [57] ABSTRACT PP N08 117,533 A fast-mode particularly a zero-mode microwave application for the heating, drying, curing or other treat- [52] Us. CL 219/1055 219/10 61 ment by microwave energy of workpieces containing [51] Int Cl H05b 2 lossy dielectric material having a fast wave microwave structure and microwave energization means Con- [58] Fleld of Search ..2l9/ 10.55, 10.61 nected to the structure to energize it Such that a re [56] References Cited gion adjacent said structure will contain a I V I predominance of zero-mode or near zero-mode elec- UNITED STATES PATENTS tric field n y- 7 3,364,294 1/1968 Garibian et al. ..219 /10.61 X 1 Claim, 7 Drawing Figures \yu MICROWAVE POWER FROM MAGNE TRON E 20 I v 20 e 2/ E l 131$ i r/2 2/ a V I Q s x i l 78 Patented May 1, 1973 3 Sheets-Sheet 1 MICROWAVE POWER \yll ll \ws llllllll| i A i I. "Huh.

min.

PEA 4% & Kai/51o Patented May 9 5 Sheets-Sheet 2 ZERO MODE WORKPIECE TRAVEL REG/ON I I I I I I II II II I II 4 I z'eUULILILJ II II IIIDI a uuu FIG. 5.

M/vavrw? 2501 w Axes/m 8 Patented May 1,1973

3 Sheets-Sheet 5 ZERO MODE 3 srr/e PHASE SHIFT PER CIRCUIT BAR( 3P 7 6 5 4 3 2 l 0 If C. 5533mm LEGENDI+ THESE MODES DOMINATE IN REGION 8 @THESE MODES DOMINATE IN REGION A oum om 1.62 55 5mm 2533M wzbjwm DISTANCE ALONG STRUCTURE (P= DISTANCE BETWEEN BARS) F/GZ ZERO-MODE MICROWAVE APPLICATOR This invention relates to a fast-mode microwave applicator and more particularly to a microwave applicator in the zero-mode of electromagnetic energy transmission.

The purpose of a microwave applicator is to couple RF energy into a lossy dielectric work load for purposes of heating, drying, curing, etc. The required microwave energy is normally taken from a standard microwave transmission line via a suitable transition. In the presence of conducting walls, e.g., microwave ovens, modified transmission lines, waveguide terminations, standing wave'sare established due to reflected energy. These standing waves cause localized heating (hot spots) which is usually undesirable..Various methods are at present used to overcome this problem. Mode stirrers are used in microwave ovens and matched loads are used to terminate travelling wave type waveguide applicators.

It is an object of the present invention to provide a microwave applicator that has an even electric field distribution and that does not produce localized heating effects.

It is another object of the invention to provide a microwave applicator that can be readily used for heating, drying, or curing of various materials such as plastic,- paper, rubber, wood materials and others materials that are well known by people versed in industrial microwave applications.

DEFINITION OF FAST MODE, SLOW MODE, AND ZERO-MODE An electromagnetic mode is said to be fast whenever its phase velocity is greater than the velocity of an electromagnetic wave in free space. On the other hand an electromagnetic mode is said to be slow whenever its phase velocity is smaller than the phase velocity of an electromagnetic wave in free space. The phase velocity (V,,,,) of a propagating mode is given'by the following relation pH /B (1) where w is the radian frequency of the wave and B is its propagation constant. Equation (1) may be written in a different form in order to better understand the physical significance between a fast and a slow" mode. Equation (2) below derives directly from equation (1) where f is the frequency of the mode and A, is the wavelength of the propagating mode.

Therefore we have for a fast" mode V,,,,),- the following relationship From equations (4), (3), and (2) we have that Mb M where is now the wavelength of the fast mode. In a similar manner we can derive the following relationship between the wavelength of a slow mode [(k, and the wavelength A,,.

(AIDS o By near zero-mode we mean a mode that is essentially close to B 0 but not necessarily at B 0. The expression fast-mode as designated above has been used in the technical literature, and in particular by J. F. Gittins in the book Power Travelling Wave Tubes the English Universities Press Limited, 102 Newgate Street, London, E. C. I. on page 194.

These and other objects of the invention are achieved by a microwave applicator operating in or near the zero mode of electromagnetic energy in periodic wave structures. The zero mode has a propagation constant equal to zero and its wavelength on a periodic wave structure is infinite. The near-zero modes have very long wavelengths and may also be used with shorter workpieces.

In drawings which illustrate embodiments of the invention, I

FIG. 1 is a cross-section of a microwave applicator arrangement for circular cross-section or tubular workpiece,

FIG. 2 is a view in cross-section of the fast-mode applicator, I r

FIG. 3 is a transverse cross-section of the apparatus of FIG. 2,

FIG. 4 is a view of a linear fast-mode applicator for flat sheet workpieces,

FIG. 5 is a plan view of the apparatus of FIG. 4, FIG. 6 is a Brillouin diagram of a fast-mode applicator structure of the type of FIGS. 1, 2, 3, and

FIG. 7 is the relative electric field strength at frequency f,, (zero-mode) in region B of the apparatus.

Referring to FIGS. 1, 2, and 3, a fast-mode microwave applicator for tubular workpieces that require heating, drying, curing, or other microwave energy treatment, has an input waveguide 10 for introduction of microwave power from a suitable power source such as a magnetron. A transition from rectangular waveguide to coaxial, in this case a doorknob type transition 11 directs the energy to a coaxial section made up of inner conductor 12 and outer conductor 13 having a length L which in practice will be much larger than shown in FIG. 1. An even number of metal (conducting) rods or tubes 15 placed on a circle of given diameter are electrically contacted near their mid-plane by circular parallel transmission lines 16 and 17 in alternate fashion as can be seen specifically in FIG. 3. The central conductor l2 of the coaxial line makes contact with circular parallel transmission line 16 (FIG. I) and the outer conductor 13 makes contact with end shorting plate 14 connected to the ends of all the rods. A second shorting plate 19 is connected to the other ends of the bars 15.

The separation of the shorting (conducting) plates 14 and 19 determines the operating frequency of the fast-mode applicator. Although more than one frequency can exist for a fast-mode in a given structure it is usually the lowest mode which is of interest. The

region indicated as B has been found to contain for this configuration well defined fast-modes including the zero-mode. A typical workpiece 22, in this case a tube of plastic extruded from plastic pellet feed hopper 20 and extruder 21 shown in symbolic form passes downward and encircling the zero-mode applicator. The fast-mode engenders a field 18 passing from transmission lines 16 and 17 and as these are unidirectional and symmetric, they pass consistently through the material. A strong heating effect is obtained by the electric field on the lossy material in the workpiece.

Because of the necessity for safety, i.e., there could be microwave leakage giving rise to adverse effects on operators, a radiation suppression 23 encircles the workpiece at the level of the circular transmission lines 16 and 17. This suppressor consists of a structure forming radial rectangular waveguides below cut-off for the frequency being employed.

FIG. 4 shows a fast-mode applicator similar to that of FIG. 1 but where the device is extended linearly. Parallel transmission lines 25 and 26 are contacted respectively by conductors 27 and 28 of alternate polarities. A fast-mode and in particular zero-mode region B exists above the transmission lines in the area shown and a flat workpiece could pass over the lines and be subjected to the zero-mode or near zero-mode fields for any wanted operation involving microwave energy. FIG. is a plan view of the apparatus of FIG. 4 but showing two sets of the transmission lines 25 and 26. This would provide a greater drying working area. The apparatus would be suitably energized from a microwave source (not shown) and the length of the microwave treatment region could be as long as desired within practised limits.

A lossy dielectric workpiece uniformly about the fast-mode applicator (FIG. 1) or on the upper surface of the linear type (FIG. 4) will contribute to adding a uniformly distributed capacitance and resistance to each periodic element of the structure. The capacitance element will have the efiect of lowering the frequency of the -zeromode or other fast-mode for a given applicator design but it cannot alter the general shape of the Brillouin diagram of the periodic structure. The resistive element of the dielectric material will lower the electric field amplitudes but it can in no way interfere with the Brillouin diagram of the structure.

It is usual practice to describe a periodic structure such as described in the above description by a Brillouin, diagram. Information regardingits pass-bands, stop bands, phase velocity, and group velocity are contained in such a diagram. From such diagrams the frequencies at which the propagation constant is zero or near zero may be identified. FIG. 6 is a measured Brillouin diagram of the structure shown in FIG. 1. At any frequency in a pass-band or at a band-edge the energy may propagate on the structure in different modes. For example at the band-edge frequencyf energy may propagate at BP 0 (zero-mode), BI 1r, 31r, etc. These modes are called the fundamental, first, second, space harmonics. The fundamental and its space harmonics each have their own amplitude and propagate simultaneously at a given frequency. It is therefore most important in a microwave applicator of the type described above that the fundamental mode (zero-mode or near zero-mode) amplitude dominate in a given space. If this is not the case the space harmonics which do have a finite wavelength would under certain conditions create well defined maximum and minimum and the problem of localized heating would again arise.

It has been found that region B of the applicator shown in FIGS. 1, 2, 3 and also FIG. 4 contains modes with little or no space harmonic content. FIG. 7 is a plot of the relative electric field strength squared for the f (zero-mode) frequency in region B. It will be seen that the field strength is close to constant and indicates that an applicator working in the zero-mode or other fast-mode would be practical and efficient.

What is claimed is:

1. A zero-mode microwave applicator for the heating, drying, or curing of workpieces containing lossy material comprising:

a. a first set of parallel conducting rods positioned in spaced relation at fixed distances in a circle,

b. a second set of parallel conducting rods interleaved alternately in the circle with the first set,

a first transmission line in the form of an annular conducting plate encircling the said rods and shaped to make electrical contact with the first set of rods,

a second transmission line in the form of an annular conducting plate parallel to the first and encircling the said rods and shaped to make electrical contact with the second set of rods,

.shorting plates connected at each end of said rods,

and, microwave energization means comprising a coaxial transmission line, the center conductor of which makes electrical contact with one of the said transmission lines and the outer conductor with one of the said shorting plates. 

1. A zero-mode microwave applicator for the heating, drying, or curing of workpieces containing lossy material comprising: a. a first set of parallel conducting rods positioned in spaced relation at fixed distances in a circle, b. a second set of parallel conducting rods interleaved alternately in the circle with the first set, c. a first transmission line in the form of an annular conducting plate encircling the said rods and shaped to make electrical contact with the first set of rods, d. a second transmission line in the form of an annular conducting plate parallel to the first and encircling the said rods and shaped to make electrical contact with the second set of rods, e. shorting plates connected at each end of said roDs, and, f. microwave energization means comprising a coaxial transmission line, the center conductor of which makes electrical contact with one of the said transmission lines and the outer conductor with one of the said shorting plates. 